With the development of radio and space technologies, several satellites based navigation systems have already been built and more will be in use in the near future. One example of such satellites based navigation systems is Global Positioning System (GPS), which is built and operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks on to this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
The GPS receiver has to acquire and lock onto at least four satellites in order to derive the position and time. Usually, a GPS receiver has many parallel channels, each receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the pseudo-random noise (PN) code phase. Each satellite transmits signals using a unique 1023-chip long PN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PRN code sequence to correlate with the digitized received satellite signal sequence. During acquisition stage, the code phase search step is a half-chip for most navigational satellite signal receivers. So the full search range of the code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from local oscillator instability.
Coherent integration and noncoherent integration are two commonly used signal integration methods to acquire GPS signals. Coherent integration provides better signal gain at the cost of larger computational load, for equal integration intervals.
The power associated with noncoherent integration of one millisecond correlation is
                    Power        =                              ∑                          n              =              0                                      N              -              1                                ⁢                      (                                                            I                  ⁡                                      (                    n                    )                                                  2                            +                                                Q                  ⁡                                      (                    n                    )                                                  2                                      )                                              (        1        )            and the power associated with coherent integration is
                    Power        =                                            (                                                ∑                                      n                    =                    0                                                        N                    -                    1                                                  ⁢                                  I                  ⁡                                      (                    n                    )                                                              )                        2                    +                                    (                                                ∑                                      n                    =                    0                                                        N                    -                    1                                                  ⁢                                  Q                  ⁡                                      (                    n                    )                                                              )                        2                                              (        2        )            where I(n) and Q(n) denote the one-millisecond correlation values from the baseband section at interval n, and N denotes the desired number of one-millisecond integration intervals.
The widely adopted criterion to declare success of acquisition is that the accumulated power exceed a preset power threshold. The power threshold setting method for signal acquisition has great impact on signal acquisition performance, including sensitivity, acquisition time, and probability of signal detection and false alarm, etc. Algorithms have been developed for setting acquisition thresholds. Published U.S. Patent Application 2003/0090414 selects the channel with the highest Signal-to-Noise ratio and sets the threshold based on this to select a set of four satellites with the highest Signal-to-Noise ratios. U.S. Pat. No. 6,125,135 derives a table of thresholds based on Carrier-to-Noise Ratio (CNR) for L1 P code acquisition but does not address C/A code acquisition. U.S. Pat. No. 6,606,349 derives the threshold based on the signal power. Finally, U.S. Pat. No. 6,643,320 has each correlator use a different threshold, the different thresholds being derived by the known distance to the satellite and the existing CNR. Thus the thresholds are functions of correlation noise level and predicted signal strength. Any satellite which is out of view or a PRN code that is not used by any of the satellites is used in a separate channel to estimate the noise level.
Whether the threshold is fixed or adaptive, one common method of these algorithms is that all satellites signals are acquired based on a single threshold except in the case of the U.S. Pat. No. 6,643,320. However, in U.S. Pat. No. 6,643,320 the antenna gain variation is not considered. It is very important to consider the transmitted power and the antenna gain in determining the acquisition threshold. The received signal power levels from satellites at different elevation angles are different. In addition, the difference in the received signal power levels may be as high as 8 dB when the gain of the receiver antenna is considered. Therefore, it is hard to find a common threshold for all satellite channels, especially for weak signals because of the small margin between signal and noise level. If the threshold is high, then the signal from low elevation satellites might not be acquired. On the other hand, if the threshold is low, then the false alarm rate for high elevation satellites increases greatly. To solve this problem, the present invention provides an elevation based adaptive threshold scheme for setting power thresholds. The present invention requires less computation than the scheme proposed in U.S. Pat. No. 6,643,320 because no distance computation between the receiver and each of the satellites is involved and the antenna gain variation is also taken into account without measuring the gain pattern. Further, U.S. Pat. No. 6,643,320 does not take into consideration the transmitter antenna gain variation with the elevation angle.